1. Field of the Invention
This invention relates to gundrills, i.e., self-piloting drills provided with an internal drilling fluid passage for drilling holes that are quite deep relative to their diameter.
2. Background Art
Gundrills are commonly used to drill deep holes into metal components, wherein the hole depth greatly exceeds the hole diameter. Gundrills are widely used in a number of industry applications, such as engine manufacturing, heat exchangers, machine tools, or the like. These applications commonly require deep holes or bores, formed in metal workpieces, by a machining operation. Gundrills are used in applications characterized by holes that are sufficiently deep such that a normal twist drill having two or more cutting edges extending generally outward from a central axis would not work effectively. When drilling relatively deep holes, twist drills tend to wander away or diverge from a central drilling axis, and the flutes tend to clog with metal chips.
In comparison, a gundrill typically has a single cutting edge eccentrically oriented off of the drill centerline. Typically, it is necessary to use a starting bushing or pilot hole to prevent the drill from orbiting off center as the hole is started. Once the hole is initially formed, the drill tip axially guides itself. Gundrills are typically provided with an elongate axial drilling fluid passageway which extends through the drill shank and drill tip. Drilling fluid or coolant is pumped through the drilling fluid passageway and as chips are formed, they are removed from the hole in a stream of drilling fluid in another passageway formed between the hole internal diameter and an elongate groove or flute formed in the gundrill.
Over the years, a number of tip geometries have been utilized in an effort to optimize a drill's performance. In spite of the years of development efforts, there remains a need for an improved gundrill design which can drill holes in a variety of metal workpieces from relatively brittle cast iron to soft ductile aluminum. The gundrill ideally, should be capable of efficiently forming small chips at the cutting tip, which can be effectively extracted from the hole by the cutting fluid. Manufacturers are always trying to increase drill feed rates while minimizing tip wear. It is desirable to maximize the number of holes that can be formed in an hour and to maximize the number of holes that can be formed before the drill tip needs to be redressed. It is further desirable from a maintenance standpoint to have a single tip design which has a minimum number of planar facets which can be redressed by a standard grinding wheel in a gundrill sharpening station.